The present invention relates to a network system, and more particularly to a network system which is designed in such a way that in the network system, in which a plurality of computers provided in one or more centers and a terminal device such as a work station or a personal computer are connected to one another through a node by telecommunication lines, on occurrence of a failure or stop in one computer of the center, or a failure or stop in the telecommunication line between the one computer and the node, a computer to be communicated with the terminal device is switched from the one computer to another computer provided in the same center or another center, and a line switching method used therein.
As for the prior art line switching method as described above, for example, there is known a method disclosed in JP-A-2-60337 corresponding to U.S. patent Ser. No. 398171. This method is arranged in such a way that in a network system in which units such as a plurality of terminal devices, host computers and exchanges (hereinafter, each being also referred to as simply "a unit", when applicable) are connected to one another through a line switching devices by telecommunication lines and back up telecommunication lines, and a network organizing center is connected to the line switching device, during a fault of the telecommunication line, an operator performs an operation of line switching indication from the remote network organizing center to each of the line switching devices, and in response to this invention, each of the line switching devices performs the operation of switching over to the back up line.
On the other hand, in JP-A-62-57337, there is disclosed a data transmission system in which, as shown in FIG. 1, a plurality of data transmission units 1021 through 1023 are connected to one another, processors 1001 and 1003 are connected to the data transmission units 1021 and 1023, respectively and an auxiliary processor 1002 is connected to the data transmission unit 1022. In this system, when the indication is given from the processor 1003 to the processor 1001 to perform the data transmission through the data transmission units 1023 and 1021, if a fault occurs in the processor 1001, the data transmission unit 1021 connected to the processor 1001 serves to change an address of the processor to transmit data from an address of the processor 1001 to that of the auxiliary processor 1002. Accordingly, thereafter, whenever the processor 1001 is designated to transmit the data from the processor 1003 to the data transmission unit 1021, that data transmission unit 1021 performs the address modification processing to transmit the data to the auxiliary processor 1002 through the data transmission unit 1022.